The new phallocystean dinoflagellate cyst genus Weiachia is described from the Toarcian of Switzerland. In contrast to the genus Moesiodinium Antonescu 1974, the new genus is characterized by an intercalary archeopyle with an adnate operculum. The type species, W. fenestrata, has openings in the periphragm which correspond in geometry and topology to paraplates. For this kind of opening a new morphological term, "fenestrae" (sing. "fenestra"), is introduced. The generic value of the free versus adnate operculum, used by Below (1987) for differentiating phallocystean dinoflagellate cyst genera, is discussed. It is a stable character for the group and is therefore considered to be a better classificatory criterion than either wall structure or the degree of expression of paratabulation by ornamentation and protuberances; both are variable characters which have been used previously in generic differentiation. The differentiation of genera using a free versus adnate operculum seems to be useful in palynostratigraphy for the late Early to early Middle Jurassic.

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